As portable electric products such as video cameras, portable phones and portable PC are more actively used, the importance of a secondary battery generally used as a driving source of such a portable electric product is increasing.
Different from a primary battery which may not be charged, a secondary battery allows charging and discharging and is actively studied in high-tech industries such as digital cameras, cellular phones, laptop computers, power tools, electric bicycles, electric vehicles, hybrid vehicles, mass-capacity power storage device or the like.
In particular, the use of a lithium secondary battery is gaining since it has a high energy density per unit weight and allows rapid charging in comparison to other existing secondary batteries such as lead storage batteries, nickel-cadmium batteries, nickel-hydrogen batteries, nickel-zinc batteries or the like.
The lithium secondary battery has an operating voltage of 3.6V or above and is used as a power source of a portable electronic device. In other cases, a plurality of batteries are connected in series or in parallel and used for high-power electric vehicles, hybrid vehicles, power tools, electric bicycles, power storage devices, UPS or the like.
The lithium secondary battery has a triple operating voltage in comparison to nickel-cadmium batteries or nickel-metal hydride batteries and is used more and more due to its high energy density per unit weight.
Depending on the kind of electrolyte, a lithium secondary battery may be classified into a lithium ion battery using a liquid electrolyte and a lithium ion polymer battery using a solid electrolyte. In addition, depending on the kind of polymer solid electrolyte, the lithium ion polymer battery may be classified into an all solid lithium ion polymer battery containing no electrolyte and a lithium ion polymer battery using a gel-type polymer electrolyte containing an electrolyte.
Lithium ion batteries using a liquid electrolyte mostly use a cylindrical or rectangular metal can as a container and are sealed therein by welding. A can type secondary battery using such a metal can as a container has a fixed shape and thus limits the design of an electric product which uses the battery as a power source. In addition, it is difficult to decrease the volume of the product. Therefore, a pouch type secondary battery prepared by putting an electrode assembly and an electrolyte into a pouch package made of a film and then sealing the same has been developed and used.
However, the lithium secondary battery may explode when being overheated, and so issues of ensuring safety are of major concern. The lithium secondary battery may be overheated due to various factors, of which an example is a case in which an overcurrent exceeding a limit flows through the lithium secondary battery. If an overcurrent flows, the lithium secondary battery generates Joule's heat and thus an internal temperature of the battery rapidly increases. In addition, the rapid increase of temperature causes a decomposition reaction of the electrolyte and thermal runaway, which may lead to explosion of the battery. An overcurrent may occur when a sharp metallic matter pierces the lithium secondary battery, when an insulation between a cathode and an anode breaks due to the shrinkage of a separator interposed between the cathode and the anode, when a rush current is applied to the battery due to an abnormal charging circuit or a load connected to the outside, or the like.
Therefore, the lithium secondary battery is coupled to a protection circuit in order to protect the battery against abnormal states such as the occurrence of an overcurrent. The protection circuit generally includes a fuse element which irreversibly disconnects a line where a charging or discharging current flows when an overcurrent occurs.
FIG. 1 is a circuit diagram for illustrating an arrangement and an operating mechanism of a fuse element employed in a protection circuit coupled to a battery pack which includes a lithium secondary battery.
As shown in FIG. 1, a protection circuit includes a fuse element 1, a sensing resistor 2 for sensing an overcurrent, a microcontroller 3 for monitoring an occurrence of an overcurrent and operating the fuse element 1 when an overcurrent occurs, and a switch 4 for switching the flow of an operating current to the fuse element 1.
The fuse element 1 is installed along a main line connected to the outermost terminal of the battery pack. The main line represents a wire through which a charging current or a discharging current flows. In FIG. 1, it is depicted that the fuse element 1 is installed along a high potential line (Pack+).
The fuse element 1 is a three-terminal element, in which two terminals are connected to the main line through which a charging or discharging current flows and one terminal is connected to the switch 4. In addition, the fuse element includes a fuse 1a connected to the main line in series and disconnected at a specific temperature and a resistor 1b for applying heat to the fuse 1a. 
The microcontroller 3 monitors the occurrence of an overcurrent by periodically detecting voltages at both ends of the sensing resistor 2, and turns on the switch 4 if it is determined that an overcurrent occurs. In this case, the current flowing through the main line flows to the fuse element 1 by bypassing sensing resistor 2 and the battery pack and is applied to the resistor 1b. Accordingly, the Joule's heat generated at the resistor 1b is conducted to the fuse 1a to raise the temperature of the fuse 1a. If the temperature of the fuse 1a increases to a melting temperature, the fuse 1a is fused to irreversibly cut the main line. If the main line is cut, the overcurrent does not flow any more, which solves problems caused from the overcurrent.
However, the above techniques have several problems. For example, if the microcontroller 3 malfunctions, even though an overcurrent occurs, the switch 4 does not turn on. In this case, a current is not applied to the resistor 1b of the fuse element 1, and therefore the fuse element 1 does not operate. In addition, a space for disposing the fuse element 1 should be separately provided in the protection circuit, and a program algorithm for controlling operations of the fuse element 1 should be loaded on the microcontroller 3. Therefore, the spatial efficiency of the protection circuit deteriorates, and the load of the microcontroller 3 increases.